Separate digest auth from HTTP headers parsing

PUBLISHED_FROM=f97569ae2f31ab94ce4875eae4d0a198f719c388

docs/c-api/http_server.h/intro.md

@@ -3,6 +3,7 @@ title: "Server API reference"
 symbol_kind: "intro"
 decl_name: "http_server.h"
 items:
+  - { name: mg_check_digest_auth.md }
   - { name: mg_file_upload_handler.md }
   - { name: mg_get_http_basic_auth.md }
   - { name: mg_get_http_header.md }

docs/c-api/http_server.h/mg_check_digest_auth.md

@@ -0,0 +1,17 @@
+---
+title: "mg_check_digest_auth()"
+decl_name: "mg_check_digest_auth"
+symbol_kind: "func"
+signature: |
+  int mg_check_digest_auth(struct mg_str method, struct mg_str uri,
+                           struct mg_str username, struct mg_str cnonce,
+                           struct mg_str response, struct mg_str qop,
+                           struct mg_str nc, struct mg_str nonce,
+                           struct mg_str auth_domain, FILE *fp);
+---
+
+Authenticates given response params against an opened password file.
+Returns 1 if authenticated, 0 otherwise.
+
+It's used by mg_http_check_digest_auth(). 
+